The present invention relates to a device for measuring visibility range, particularly for motor vehicles, which has at least one optical transmitting element, at least one optical receiving element and a measuring-signal evaluation unit which ascertains an instantaneous visibility range from the light reflected from one or more space zones which are at different distances.
Such a device is known from German Patent No. DE 196 29 712 This known device measures backscattering according to the LIDAR principle. In this case, a light transmitter (e.g. a laser or a photodiode) emits light pulses into the surroundings located in front of a vehicle. Depending on the scattering medium (e.g., fog, snow, rain, smoke, etc.) in the surroundings in front of the vehicle, a greater or lesser portion of the light is reflected after the interaction with the scattering medium, and received by one or more receiving elements (e.g. photodiode). The visibility range is ascertained from the relationship of emitted light to reflected light. However, there are several interference effects on the transmission path of the light which influence the received signals, and thus invalidate the visibility-range measuring result. Sources of error when measuring the visibility range are, for example, a change in the transmitting power of the emitted light because of temperature changes or aging of the transmitting element, for instance, or soiling of the optical system pre-connected to the transmitting and receiving elements, or soiling of the motor-vehicle windshield through which the emitted and received light pulses pass. To eliminate these interference effects, according to the related art mentioned, the backscatter light is determined from two space zones which are at different distances. The indicated interference effects can be eliminated by forming the ratio of the received signals.
It turns out that the measuring signals received by the optical receiver are very strongly dependent on the backscattering properties, i.e. on the type of scattering medium. For example, in the case of fog, the backscatter characteristics are dependent on the droplet-size distribution. However, an unadulterated visibility-range measurement is only possible when the type of scattering medium has no influence on the visibility-range measurement. Therefore, an object of the present invention is to provide a device of the type indicated at the outset which permits a visibility-range measurement that is independent of the type of scattering medium.
The indicated objective is achieved, in that means are provided which normalize a plurality of successively recorded measuring curves, each of which is composed of a plurality of measured values of light reflected from space zones at different distances. The means form an average measuring curve from the normalized measuring curves, and derive from at least one measured value of the averaged measuring curve, a visibility range which an evaluation unit uses for the adaptive compensation (adjustment) of an instantaneously ascertained visibility range. The instantaneous visibility range is determined according to the related art from the light reflected from one or more space zones that are at different distances.
The instantaneous visibility range is measured without great time delay, and can be implemented repeatedly within a second. The instantaneous visibility range is compensated according to the present invention using a measured visibility range resulting from a longer-lasting averaging process of past measured values, with the goal of eliminating the influence of the scattering-medium characteristics on it. The averaging process for determining the visibility range used as compensation makes it superfluous to send out light signals having very high levels which represent a danger for the eyes. The ascertainment of the visibility range that is independent of the scattering medium, which is more tedious because of the averaging process, does not delay the actual measurement of an unadulterated visibility range, since it is merely used as a compensation variable for the instantaneous visibility range that is determined very quickly, but not without influence by the type of scattering medium.
Accordingly, it is expedient that the means normalize all measuring curves, recorded in succession over time, to the maximum value of the measuring curves.
A classifier, operating according to the fuzzy principle, advantageously divides the respectively ascertained, compensated visibility ranges into distance classes.
The means advantageously set a plurality of measured values from the averaged measuring curve in relation to one another, and derive the visibility range therefrom.
Additional means can be provided which acquire from the visibility range, which has been derived from the averaged measuring curve, information about the type of medium impairing the visibility range.
The present invention shall now be explained in greater detail.